Experiments with many systems suggest that each cell knows its relative position in its developing "field", and that this positional information controls the determined state of the cell. Although the cell surface appears to be involved, there is very little known about the detailed cellular or molecular events underlying the intercellular signals that establish and maintain the positional fields. As many diseases or congenital abnormalities involve breakdowns in the cellular interactions that regulate growth or differentiation, an understanding of the mechanisms of these interactions will further our understanding of these pathological states. The embryonic cells of Drosophila (including the imaginal cells of the larva) represent one of the best characterized and most tractable systems for examining the development of undifferentiated cells. Initially, we will make monoclonal antibodies against imaginal discs. The antibodies will be screened by immunofluorescence against a variety of larval tissues, and the distributions of fluorescences will be compared to other information concerning the determined states of the undifferentiated cells to select antibodies which are likely to be against cell surface antigens of developmental significance. Selected antibodies will be used in more extensive immunofluorescent analyses--both to characterize the antibodies and to learn more about various aspects of Drosophila development. Selected antibodies will also be employed in a functional assay designed to define antigens that are involved in the intercellular signalling that alters the regulative potential of fragments of imaginal discs. The functions of antigens will also be examined by genetic analysis, using the antibodies to screen for variants as an initial step in the mapping and characterization of the genes. And, we will characterize the antigens biochemically, using immunoabsorbent techniques for their isolation. The biochemical work will also be combined with the functional analyses to define functional domains within the molecules. In summary, we will examine the surface antigens of undifferentiated, essentially embryonic cells. Our approach will allow us to define and characterize molecules involved in pattern regulation and early determinative events. Using the wide variety of techniques available with Drosophila, it should eventually be possible to define, in molecular terms, how these molecules function and how their expression is controlled.